This application claims priority of European Patent Application No. 99309308.7, which was filed on Nov. 23, 1999.
This invention relates to mobile telecommunications systems, particularly though not exclusively the Universal Mobile Telecommunications System (UMTS).
Packet transmission is a key feature of the UMTS as defined by the 3GPP(trademark) standard. Methods to make the packet transmission efficient are required on both Downlink and Uplink. On the Downlink the radio access network has complete knowledge of how much, when, and to whom the packet transmissions are made. Hence a central control can be used for the Downlink packet transmission, for example, through a scheduler in the radio access network side to schedule the Downlink packet transmission.
For Uplink packet transmission where all mobile users are uncoordinated, a fully central controlled method (e.g., scheduling based method) on the Uplink is not feasible. There is therefore a procedure set forth in the MAC protocol specification for UMTS for gaining transmission access by making random packet transmissions on a random access channel (RACH) and detecting, according to a CDMA (Collision Detect Multiple Access) process, whether the packet transmission is received and acknowledged by the receiver. If the transmission is unsuccessful, the transmitter makes another attempt after a random time interval. The probability of gaining access is dependent on the number of other stations making transmissions, and, where a station is unsuccessful, the time interval for making another attempt is lengthened according to a persistency value, which is derived by the Radio Access Network from the traffic volume. Thus, Uplink packet transmission may be controlled through persistency broadcast on the Downlink to the User Equipment (UE). Persistency probability is updated and available for Uplink packet transmissions on the Random Access Channel and Common Packet Channel (RACH/CPCH).
Hence the principle of persistency control is that the radio access network monitors the Uplink traffic load and broadcasts a persistency value on the downlink Broadcast Control Channel. Based on the persistency broadcast information, UEs decide their (re) transmission probability, so that the packet access on RACH/CPCH is controlled.
The 3GPP Standard for UMTS is however silent as to the implementation of persistency control.
It is an object of the present invention to provide a practical and efficient persistency control algorithm for a mobile telecommunications system, based on specific measurement values provided by the system, and which avoids tedious computation.
In a first general aspect, the present invention provides, in a mobile telecommunications system, a method of determining a persistency value for making random transmissions for access to the system, the method comprising:
1. determining for a system cell, the maximum allowable user equipment transmission power PMAX; characterised by:
2. assessing the number, in a current time interval, of ongoing packet transmissions NCURRENT based upon a fraction a of measured current transmission power Ptotal;
3. assessing the power available in a next time interval based upon the maximum allowable transmission power and the current transmission power, and determining therefrom a number of allowable new packets NNEXT in the next time interval; and
4. determining a persistency value p for transmission to user equipment in said cell, being a function of the number ongoing packet transmissions NCURRENT and said number of allowable new packets NNEXT.
In a second general aspect, the invention provides in a mobile telecommunications system, apparatus for determining a persistency value for making random transmissions for access to the system, the apparatus comprising:
means for providing for a system cell, the maximum allowable user equipment transmission power PMAX;
means for measuring user equipment current transmission power Ptotal; characterised by:
means for assessing the number in a current time interval of ongoing packet transmissions NCURRENT based upon a fraction xcex1 of measured current transmission power Ptotal;
means for assessing the power available in a next time interval based upon the maximum allowable transmission power and the current transmission power, and determining therefrom a number of allowable new packets NNEXT in the next time interval; and
means for determining a persistency value p for transmission to user equipment in said cell, being a function of said number of ongoing packet transmissions NCURRENT and said number of allowable new packets NNEXT.
In a more specific aspect, the invention provides in the Universal Mobile Telecommunications System, a method of determining a persistency value for making random transmissions for access to the system, the method comprising;
1. determining for a system cell, the maximum allowable user equipment transmission power PMAX; characterised by:
2. assessing the number in a current time interval of ongoing packet transmissions NCURRENT based upon a fraction xcex1 representing the user equipment measured current transmission power Ptotal; wherein Ptotal includes power from both packet Transport Channels (xcex1.Ptotal) and power from circuit traffic Dedicated Transport Channels((1xcex1).Ptotal.)
3. assessing the power available in a next time interval based upon the maximum allowable transmission power and the current transmission power, and determining therefrom a number of allowable new packets NNEXT in the next time interval; and
4. determining a persistency value p for transmission to user equipment in said cell, being a function of said number of ongoing packet transmissions NCURRENT and said number of allowable new packets NNEXT.
Said fraction xcex1 may have any value of one or less than one. As preferred, said persistency value p is determined on the assumption that all ongoing packet transmissions NCURRENT will continue in the next time interval. Said persistency value p is preferably determined on the assumption that all ongoing packet transmissions NCURRENT and all said number of allowable new packets NNEXT have an equal possibility of transmission. In a preferred arrangement, the normalised persistency value p is equal to the inverse of the sum of said number in a current time interval of ongoing packet transmissions NCURRENT and said number of allowable new packets NNEXT. (note that p is normalised value.)
The number of ongoing packet transmissions NCURRENT is determined on the assumption that each packet is controlled to have the same power PRACH, whereby NCURRENT=xcex1.Ptotal/PRACH. The number of allowable new packets NNEXT is determined on the assumption that each packet has the same power (at receiver side) PRACH, whereby NNEXT=(PMAXxe2x88x92Ptotal)/PRACH. Thus, the persistency value p is given by PRACH(PMAX+(1xe2x88x92xcex1).Ptotal)xe2x88x921.
An overall persistency probability is transmitted over the system cell, having been determined as the product of the normalised persistency value p and the number n of total access channels available for packet transmission.
For the Universal Mobile Telecommunications System measured current transmission power Ptotal includes power from both packet Transport Channels (xcex1xc2x7Ptotal) and power from circuit traffic Dedicated Transport Channels((1xe2x88x92xcex1).Ptotal.)